Recognition proteins, such as antibodies, have many applications in medicine and biotechnology. Many of these applications would benefit from the ability to control, or activate, when the recognition protein binds to the target. This proposal seeks to develop and test a library of activatable antibody-inspired recognition proteins, or actibodies. Specifically, we first aim to introduce diversity into a regulated protein and develop a new display system and accompanying screening technology that is appropriate for this protein. We will then determine how well individual variants selected in this fashion can recognize the target molecules. In our second aim, we will determine whether members of this library are indeed actibodies, that can be activated to bind the target. This work will help us understand a newly discovered type of allosteric regulation, and determine whether it is useful for making regulated recognition proteins. The resulting actibody technology should contribute to a range of biomedical technologies, including targeted drug delivery, molecular imaging, in vitro diagnostics, and separation technologies. In particular, the ability to activate binding should enhance the ability to specifically identify only the correct target in all applications, and would also allow for gentle recovery of the target in the in vitro applications.